U.S. patent application number 14/414227 was filed with the patent office on 2015-07-30 for vertically aligned arrays of carbon nanotubes formed on multilayer substrates.
The applicant listed for this patent is Carbice Nanotechnologies, Inc.. Invention is credited to Baratunde A. Cola.
Application Number | 20150209761 14/414227 |
Document ID | / |
Family ID | 48914416 |
Filed Date | 2015-07-30 |
United States Patent
Application |
20150209761 |
Kind Code |
A1 |
Cola; Baratunde A. |
July 30, 2015 |
Vertically Aligned Arrays of Carbon Nanotubes Formed on Multilayer
Substrates
Abstract
Multilayer substrates for the growth and/or support of CNT
arrays are provided. These multilayer substrates both promote the
growth of dense vertically aligned CNT arrays and provide excellent
adhesion between the CNTs and metal surfaces. Carbon nanotube
arrays formed using multilayer substrates, which exhibit high
thermal conductivity and excellent durability, are also provided.
These arrays can be used as thermal interface materials.
Inventors: |
Cola; Baratunde A.;
(Atlanta, GA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carbice Nanotechnologies, Inc. |
Atlanta |
GA |
US |
|
|
Family ID: |
48914416 |
Appl. No.: |
14/414227 |
Filed: |
July 10, 2013 |
PCT Filed: |
July 10, 2013 |
PCT NO: |
PCT/US13/49900 |
371 Date: |
January 12, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13546827 |
Jul 11, 2012 |
|
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14414227 |
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Current U.S.
Class: |
252/71 ;
427/249.6; 428/408; 428/606; 502/178; 502/184; 502/300; 502/318;
502/337; 502/340; 502/345; 502/355 |
Current CPC
Class: |
Y10S 977/81 20130101;
B01J 23/44 20130101; Y10T 428/265 20150115; B01J 21/02 20130101;
C09K 5/14 20130101; B01J 23/466 20130101; B01J 35/02 20130101; B82Y
40/00 20130101; B01J 35/0006 20130101; C01B 32/162 20170801; B01J
23/42 20130101; C01B 2202/08 20130101; Y10S 977/742 20130101; B01J
23/468 20130101; B01J 23/464 20130101; Y10T 428/12611 20150115;
B01J 23/72 20130101; B01J 23/75 20130101; Y10T 428/12493 20150115;
B01J 23/745 20130101; B01J 23/755 20130101; Y10T 428/12431
20150115; Y10T 428/24975 20150115; B82Y 30/00 20130101; Y10T
428/12576 20150115; Y10T 428/30 20150115; Y10T 428/25 20150115 |
International
Class: |
B01J 23/745 20060101
B01J023/745; B01J 21/02 20060101 B01J021/02; C01B 31/02 20060101
C01B031/02; B01J 35/00 20060101 B01J035/00; B01J 35/02 20060101
B01J035/02; B01J 23/72 20060101 B01J023/72; C09K 5/14 20060101
C09K005/14 |
Claims
1. A multilayer substrate for the growth and/or support of a
plurality of carbon nanotubes comprising an inert support; one or
more adhesion layers; one or more interface layers; and a catalytic
layer.
2. The substrate of claim 1, comprising an inert support, a first
adhesion layer, a first interface layer, and a catalytic layer.
3. The substrate of claim 1, comprising an inert support, a first
adhesion layer, a first interface layer, a second adhesion layer, a
second interface layer, and a catalytic layer.
4. The substrate of claim 1, wherein the inert support is a metal
selected from the group consisting of aluminum, platinum, gold,
nickel, iron, tin, lead, silver, titanium, indium, copper, or
combinations thereof.
5. The substrate of claim 1, wherein the inert support is a metal
alloy.
6. The substrate of claim 5, wherein the alloy is copper-tungsten
pseudoalloy, diamond in copper-silver alloy matrix, or combinations
thereof.
7. The substrate of claim 1, wherein the support is selected from
the group consisting of silicon carbide in an aluminum matrix,
beryllium oxide in beryllium matrix, or combinations thereof.
8. The substrate of claim 1, wherein the adhesion layer comprises a
metal or metal alloy.
9. The substrate of claim 8, wherein the metal or metal alloy is a
transition metal or transition metal alloy that is a catalyst for
CNT formation.
10. The substrate of claim 9, wherein the metal or metal alloy is
selected from the group consisting of iron, iron alloy, nickel,
nickel alloy, or combinations thereof.
11. The substrate of claim 10, wherein the metal or metal alloy is
iron or iron alloy.
12. The substrate of claim 1, wherein the interface layer comprises
a metal selected from the group consisting of aluminum, titanium,
gold, copper, silver, tantalum, and combinations thereof.
13. The substrate of claim 1, wherein the interface layer comprises
a metal oxide selected from the group consisting of aluminum oxide,
silicon oxide, titanium dioxide, or combinations thereof.
14. The substrate of claim 1, wherein the catalytic layer is
selected from the group consisting of iron, nickel, cobalt,
rhodium, palladium, osmium, iridium, platinum, and combinations
thereof.
15. The substrate of claim 14, wherein the catalytic layer is
iron.
16. The substrate of claim 1, wherein the adhesion layer and the
catalytic layer have the same chemical composition.
17. The substrate of claim 1, wherein the adhesion layer is between
about 10 nm and about 150 nm in thickness, more preferably between
about 10 nm and about 100 nm in thickness, more preferably between
about 10 nm and about 75 nm in in thickness, most preferably
between about 15 nm and about 50 nm in thickness.
18. The substrate of claim 1, wherein the interface layer is
between about 5 nm and about 50 nm in thickness, more preferably
between about 7 nm and about 30 nm in thickness, most preferably
between about 7 nm and about 15 nm in thickness.
19. The substrate of claim 1, wherein the catalytic layer is
between about 10 nm and about 1 nm in thickness, more preferably
between about 5 nm and about 1 nm in thickness, more preferably
between about 2 nm and about 5 nm in thickness.
20. The substrate of claim 1, wherein the adhesion layer is about
30 nm in thickness, the interface layer is about 10 nm in
thickness, and the catalytic layer is about 3 nm in thickness.
21. The substrate of claim 2, wherein the first interface layer is
between about 50 nm and about 150 nm in thickness, more preferably
between about 80 nm and about 120 nm in thickness.
22. The substrate of claim 21, wherein the second interface layer
is between about 5 nm and about 50 nm in thickness, more preferably
between about 7 nm and about 30 nm in thickness.
23. The substrate of claim 21, wherein the first adhesion layer is
about 10 nm in thickness, the first interface layer is about 100 nm
in thickness, the second adhesion layer is about 30 nm in
thickness, the second interface layer is about 10 nm in thickness,
and the catalytic layer is about 3 nm in thickness.
24. An array of carbon nanotubes comprising a metal surface; an
adhesion layer; a plurality of metal oxide nanoparticles or
aggregates deposited on the adhesion layer; a plurality of
catalytic nanoparticles or aggregates deposited on the metal oxide
nanoparticles or aggregates; and a plurality of vertically aligned
carbon nanotubes attached to the catalytic nanoparticles.
25. The array of claim 24, wherein the nanotubes are present at a
density from about 1.times.10.sup.7 to 1.times.10.sup.11 nanotubes
per mm.sup.2, more preferably from about 1.times.10.sup.8 to
1.times.10.sup.10 nanotubes per mm.sup.2, most preferably from
about 1.times.10.sup.9 to 1.times.10.sup.10 nanotubes per mm.sup.2
on the inert support.
26. The array of claim 24, wherein at least 90%, 95%, 96%, 97%,
98%, 99%, or 99.9% of the CNTs remain on the surface after
sonication in ethanol.
27. The array of claim 24, further comprising one or more polymers
absorbed to the distal ends of the carbon nanotubes.
28. The array of claim 24, further comprising one or more metal
nanoparticles absorbed to the distal ends of the carbon
nanotubes.
29. The array of claim 24, further comprising a flowable or phase
change material in the space between carbon nanotubes.
30. The array of claim 24, wherein the morphology of the array is
modified by evaporating a liquid in which the array was
immersed.
31. An array of vertically aligned carbon nanotubes prepared by a
process comprising (a) annealing a multilayer substrate comprising
an inert support, an adhesion layer, an interface layer, and a
catalytic layer; and (b) heating the multilayer substrate to a
growth temperature of between 550.degree. C. and 660.degree. C.;
and (c) introducing a carbon source gas.
32. A method of improving the transfer of heat from a heat source
to a heat sink, comprising placing or affixing in between the heat
source and the heat sink an array of carbon nanotubes comprising a
metal surface, an adhesion layer, a plurality of metal oxide
nanoparticles or aggregates deposited on the adhesion layer, a
plurality of catalytic nanoparticles or aggregates deposited on the
metal oxide nanoparticles or aggregates, and a plurality of
vertically aligned carbon nanotubes attached to the catalytic
nanoparticles.
33. A method of forming an array of vertically aligned carbon
nanotubes comprising (a) annealing a multilayer substrate
comprising an inert support, an adhesion layer, an interface layer,
and a catalytic layer; and (b) heating the multilayer substrate to
a growth temperature of between 550.degree. C. and 660.degree. C.;
and (c) introducing a carbon source gas.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Ser. No. 13/546,827
entitled "Vertically Aligned Arrays of Carbon Nanotubes Formed on
Multilayer Substrates" filed on Jul. 11, 2012. The contents of this
application are incorporated herein by reference in their
entirety.
FIELD OF THE INVENTION
[0002] The invention is generally in the field of substrates for
the growth of carbon nanotube (CNT) arrays, arrays of aligned CNTs,
as well as methods of making and using thereof.
BACKGROUND OF THE INVENTION
[0003] Carbon nanotubes (CNTs) possess a variety of useful
properties, including high thermal conductivity, tensile strength,
and elastic modulus. Carbon nanotubes have been investigated for
applications in nanotechnology, electronics, optics, and other
fields of materials science and technology.
[0004] CNTs exhibit high thermal conductivity, with multi-wall
carbon nanotubes (MWCTs) exhibiting thermal conductivities up to
about 3,000 W/mK at room temperature, and single-wall carbon
nanotubes (SWNTs) exhibiting thermal conductivities up to about
5,000 to about 8,000 W/mK at room temperature. As a result, CNTs,
especially vertically aligned arrays of CNTs, have attracted
significant interest for use in thermal interface materials (TIMs).
In order to function efficiently and maintain performance over
time, the CNTs should be well anchored to a support structure,
uniformly aligned, preferably perpendicular to the support surface,
and be present at a high density on the support structure.
[0005] However, in spite of the tremendous potential of such
materials, it has proven difficult to form dense and well aligned
CNT arrays on metal surfaces, and to achieve good adhesion between
the metal and CNTs. Typically, arrays of aligned carbon nanotubes
are grown from surfaces containing a thin film (<1 nm thick) of
catalyst, such as iron, supported on a metal oxide film, such as
alumina, with a thickness of between 10 and 200 nm. Under growth
conditions, the catalyst forms small (<10 nm) islands or
particles on the surface of the oxide film from which the nanotubes
grow. The catalyst particles pack on the surface, constraining the
nanotube growth to a direction perpendicular to the surface.
[0006] Unfortunately, CNT arrays grown from these surfaces display
limited density and yield. This is the result of migration of the
catalyst particles into the oxide film during the course of
nanotube growth. See, for example, Amama, P. B. et al. ACS Nano,
4:895-904 (2010) and Kim, S. M. et al. J. Phys. Chem. Lett.
1:918-922 (2010). In addition, CNT arrays formed in this fashion
are generally poorly adhered to underlying metal surfaces, because
the oxide layer, incorporated to promote dense and aligned CNT
growth, does not adhere well to the underlying metal surface.
[0007] In order to provide improved CNT arrays for use as thermal
interface materials (TIMs), CNT arrays with higher nanotube density
and improved nanotube adhesion are required.
[0008] Therefore, it is an object of the invention to provide
surfaces for the growth of high density arrays of carbon nanotubes,
and methods of use thereof.
[0009] It is a further object of the invention to provide arrays of
vertically aligned CNTs which are well adhered to a surface, such
as a metallic surface.
[0010] It is also an object of the invention to provide arrays of
vertically aligned arrays of CNTs for use as thermal interface
materials (TIMs).
SUMMARY OF THE INVENTION
[0011] Multilayer substrates for the growth and/or support of CNT
arrays are provided. Multilayer substrates promote the growth of
dense vertically aligned CNT arrays and provide excellent adhesion
between the CNTs and metal surfaces.
[0012] The multilayer substrates contain three or more layers
deposited on an inert support, such as a metal surface. Generally,
the multilayer substrate contains one or more adhesion layers, one
or more interface layers, and one or more catalytic layers,
deposited on the surface of an inert support. Generally, the
support is formed at least in part from a metal, such as aluminum,
platinum, gold, nickel, iron, tin, lead, silver, titanium, indium,
copper, or combinations thereof. In certain instances, the support
is a metallic foil, such as aluminum or copper foil. The support
may also be a surface of a device, such as a conventional heat sink
or heat spreader used in heat exchange applications.
[0013] The adhesion layer is formed of a material that improves the
adhesion of the interface layer to the support. In certain
embodiments, the adhesion layer is a thin film of iron. Generally,
the adhesion layer must be thick enough to remain a continuous film
at the elevated temperatures used to form CNTs. The adhesion layer
also generally provides resistance to oxide and carbide formation
during CNT synthesis at elevated temperatures.
[0014] The interface layer is preferably formed from a metal which
is oxidized under conditions of nanotube synthesis or during
exposure to air after nanotube synthesis to form a suitable metal
oxide. Examples of suitable materials include, but are not limited
to, aluminum. Alternatively, the interface layer may be formed from
a metal oxide, such as aluminum oxide or silicon oxide. Generally,
the interface layer is thin enough to allow the catalytic layer and
the adhesion layer to diffuse across it. In some embodiments
wherein the catalytic layer and the adhesion layer have the same
composition, this reduces migration of the catalyst into the
interface layer, improving the lifetime of the catalyst during
nanotube growth.
[0015] The catalytic layer is typically a thin film formed from a
transition metal that can catalyze the formation of carbon
nanotubes via chemical vapor deposition. Examples of suitable
materials that can be used to form the catalytic layer include, but
are not limited to, iron, nickel, cobalt, rhodium, palladium, and
combinations thereof. In some embodiments, the catalytic layer is
formed of iron. The catalytic layer is of appropriate thickness to
form catalytic nanoparticles or aggregates under the annealing
conditions used during nanotube formation.
[0016] CNT arrays containing a plurality of vertically aligned CNTs
on a material are also provided. The CNTs are well anchored to the
material, and are present in a high density.
[0017] In some embodiments, the CNT array is formed by vertically
aligning a plurality of CNTs on the multilayer substrate. This can
be accomplished, for example, by transferring an array of CNTs to
the distal ends of CNTs grown on the multilayer substrate. In some
embodiments, tall CNT arrays are transferred to the distal ends of
very short CNTs on the multilayer substrate. This technique
improves the bond strength by increasing the surface area for
bonding.
[0018] In other embodiments, the multilayer substrate serves as a
catalytic surface for the growth of a CNT array. In these
instances, the process of CNT growth using chemical vapor
deposition alters the morphology of the multilayer substrate.
Specifically, upon heating, the interface layer is converted to a
metal oxide, and forms a layer or partial layer of metal oxide
nanoparticles or aggregates deposited on the adhesion layer. The
catalytic layer similarly forms a series of catalytic nanoparticles
or aggregates deposited on the metal oxide nanoparticles or
aggregates. During CNT growth, CNTs form from the catalytic
nanoparticles or aggregates. The resulting CNT arrays contain CNTs
anchored to an inert support via an adhesion layer, metal oxide
nanoparticles or aggregates, and/or catalytic nanoparticles or
aggregates.
[0019] The metal oxide nanoparticles or aggregates typically
contain a metal oxide formed from the metal or metals used to form
the interface layer. For example, in embodiments where the
interface layer is formed from aluminum, the metal oxide
nanoparticles or aggregates are formed from aluminum oxide. The
catalytic nanoparticles or aggregates may be composed of the metal
used to form the catalytic layer.
[0020] Generally, the nanotubes are present at a sufficient density
such that the nanotubes are self-supporting and adopt a
substantially perpendicular orientation to the surface of the
multilayer substrate. Preferably, the nanotubes are spaced at
optimal distances from one another and are of uniform height to
minimize thermal transfer losses, thereby maximizing their
collective thermal diffusivity.
[0021] The CNTs display strong adhesion to the multilayer
substrate. In certain embodiments, the CNT array will remain
substantially intact after being immersed in a solvent, such as
ethanol, and sonicated for a period of at least five minutes.
[0022] In one embodiment, the multilayer substrate is three
layered. In some embodiments, the three layered substrate is formed
from an adhesion layer (e.g., iron) of about 30 nm in thickness, an
interface layer (e.g., aluminum or alumina) of about 10 nm in
thickness, and a catalytic layer (e.g., iron) of about 3 nm in
thickness deposited on a metal surface. In this embodiment, the
iron adhesion layer adheres to both the metal surface and the Al
(alumina nanoparticles or aggregates after growth) or
Al.sub.2O.sub.3 interface layer. The iron catalytic layer forms
iron nanoparticles or aggregates from which CNTs grow. These iron
nanoparticles or aggregates are also bound to the alumina
below.
[0023] In another embodiment, the multilayer substrate is five
layered. In some embodiments, the five layered substrate is formed
from a first adhesion layer (e.g., iron) of about 10 nm in
thickness, a first interface layer (e.g., aluminum or alumina) of
about 100 nm in thickness, a second adhesion layer (e.g., iron) of
about 30 nm in thickness, and a second interface layer (e.g.,
aluminum or alumina) of about 10 nm in thickness, and a catalytic
layer (e.g., iron) of about 3 nm in thickness deposited on a metal
surface.
[0024] As a result, well bonded interfaces exist on both sides of
the oxide interface materials. Of metal/metal oxide interfaces, the
iron-alumina interface is known to be one of the strongest in terms
of bonding and chemical interaction. Further, metals (e.g., the
iron adhesion layer and the metal surface) tend to bond well to
each other because of strong electronic coupling. As a consequence,
the CNTs are strongly anchored to the metal surface.
[0025] Further, subsurface diffusion of iron from the catalytic
layer during nanotube growth is reduced because the same metal is
on both sides of the oxide support, which balances the
concentration gradients that would normally drive diffusion.
Therefore, catalyst is not depleted during growth, improving the
growth rate, density, and yield of nanotubes in the array.
[0026] The CNT arrays described herein can be used as thermal
interface materials. The CNT arrays can be formed and/or deposited,
as required for a particular application.
[0027] For example, in one embodiment, the inert support for the
CNT array is piece of metal foil, such as aluminum foil. In these
cases, CNTs are anchored to a surface of the metal foil via an
adhesion layer, metal oxide nanoparticles or aggregates, and
catalytic nanoparticles or aggregates. In some instances only one
surface (i.e., side) of the metal foil contains an array of aligned
CNTs anchored to the surface. In other cases, both surfaces (i.e.,
sides) of the metal foil contain an array of aligned CNTs anchored
to the surface. If desired one or more polymers may be applied to
the CNT array. The CNT array may also be decorated with one or more
types of metal nanoparticles. Polymers and metal nanoparticles may
be applied together to the CNT array Immersing the arrays in liquid
and then evaporating the liquid such that capillary forces during
drying change the local and or global morphology of the CNTs may
also be used to modify the CNT arrays. In other embodiments, a
flowable or phase change material may be added to the CNT arrays to
fill the space between the CNTs. These materials may be placed or
affixed in between a heat source and a heat sink or heat spreader,
such as between an integrated circuit package and a finned heat
exchanger, to improve the transfer of heat from the heat source to
the heat sink or heat spreader.
[0028] In other embodiments, the inter support for the CNT array is
a surface of a conventional metal heat sink or heat spreader. In
these cases, CNTs are anchored to a surface of the heat sink or
heat spreader via an adhesion layer, metal oxide nanoparticles or
aggregates, and catalytic nanoparticles or aggregates. This
functionalized heat sink or heat spreader may then be abutted or
adhered to a heat source, such as an integrated circuit
package.
[0029] The CNT arrays described herein can be used as thermal
interface materials in personal computers, server computers, memory
modules, graphics chips, radar and radio-frequency (RF) devices,
device burn-in testing systems, disc drives, displays, including
light-emitting diode (LED) displays, lighting systems, automotive
control units, power-electronics, batteries, communications
equipment, such as cellular phones, thermoelectric generators, and
imaging equipment, including MRIs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a cross section of a multilayer substrate for the
formation and/or support of carbon nanotube arrays.
[0031] FIG. 2 is a cross section of a carbon nanotube array formed
by chemical vapor deposition on a multilayer substrate. For
clarity, only a single nanotube, catalytic nanoparticle or
aggregate, and metal oxide nanoparticle or aggregate are
illustrated.
[0032] FIG. 3 is a diagram showing the transfer printing of long
carbon nanotubes onto an array of short carbon nanotubes.
[0033] FIG. 4 is a schematic showing the change in morphology when
a CNT array is immersed in a liquid. The SEM image shows the
aggregation of CNTs into discrete islands due to the capillary
action of the solvent evaporation.
[0034] FIG. 5 is a schematic showing CNT arrays immersed in
flowable of phase change materials.
[0035] FIG. 6 is a diagram showing the distal ends of the CNT
arrays on both sides of the aluminum foil coated with P3HT. The
polymer-coated sample is adhered to gold-coated silver and quartz
surfaces.
[0036] FIG. 7 is a graph showing the measured voltage (V) as a
function of current (Amperes) at 180.degree. C. for CNT on copper,
CNT on aluminum, and Grafoil.RTM..
DETAILED DESCRIPTION OF THE INVENTION
I. Definitions
[0037] "Thermal Interface Material" (TIM), as used herein, refers
to a material or combination of materials that provide high thermal
conductance and mechanical compliance between a heat source and
heat sink or spreader to effectively conduct heat away from a heat
source.
[0038] "Carbon Nanotube Array" or "CNT array", as used herein,
refers to a plurality of carbon nanotubes which are vertically
aligned on a surface of a material. Carbon nanotubes are said to be
"vertically aligned" when they are substantially perpendicular to
the surface on which they are supported or attached. Nanotubes are
said to be substantially perpendicular when they are oriented on
average within 30, 25, 20, 15, 10, or 5 degrees of the surface
normal.
II. Multilayer Substrates
[0039] Multilayer substrates for the formation of carbon nanotube
arrays promote the growth of dense vertically aligned CNT arrays
and provide excellent adhesion between the CNTs and metal surfaces.
Multilayer substrates also promote high CNT growth rates on metal
surfaces. Multilayer substrates contain three or more metallic thin
films deposited on the surface of an inert, preferably metallic
support.
[0040] An exemplary multilayer substrate (100) is shown in FIG. 1.
The multilayer substrate contains three layers (an adhesion layer,
104; an interface layer, 106; and a catalytic layer, 108) deposited
on the surface of an inert support (102).
[0041] A. Supports
[0042] A variety of materials can serve as a support for multilayer
substrates. Generally, the support is inert, meaning that the
support does not chemically participate in the formation of
nanotubes on the multilayer substrate.
[0043] Generally, the support is formed at least in part from a
metal, such as aluminum, cobalt, chromium, zinc, tantalum,
platinum, gold, nickel, iron, tin, lead, silver, titanium, indium,
copper, or combinations thereof and/or one or more metal oxides,
such as oxides of the metals listed above. Other materials include
ceramics and silicon or silicon compounds, such as silicon
dioxide.
[0044] In some instances, the support is a readily deformable
and/or flexible sheet of solid material. In certain embodiments,
the support is a metallic foil, such as aluminum foil or copper
foil.
[0045] The support may also be a surface of a device, such as a
conventional heat sink or heat spreader used in heat exchange
applications. Such heat sinks may be formed from a variety of
materials including copper, aluminum, copper-tungsten pseudoalloy,
AlSiC (silicon carbide in an aluminum matrix), Dymalloy (diamond in
copper-silver alloy matrix), and E-Material (beryllium oxide in
beryllium matrix).
[0046] In some embodiments, the surface of the support may be
treated to increase adhesion with the adhesion layer. Such
treatment may include the use of plasma-assisted or chemical-based
surface cleaning. Another treatment would include the deposition of
a metal or metal oxide coating or particles on the support.
[0047] Multilayer substrates can be formed on one or more surfaces
of a suitable support. For example, in certain embodiments, the
support is a metallic foil. In these instances, multilayer
substrates can be formed on one or both sides of the metallic foil
as required for a particular application.
[0048] The support, and conditions under which the CNTs are formed,
should be selected such that the support resists reacting with the
catalyst, process gases, and/or residual gases through reactions,
such as oxidation, silicidation, alloying, and/or carbide
formation. For example, C, O, H, and N are the elements composing
most CNT CVD process and contamination gases. Under certain
conditions, the support can react to form oxides, carbides, and
other byproducts which significantly reduce CNT growth which in
turn leads to loss of electrical conduction in the support.
Reaction conditions, such as temperature, can be selected in order
to minimize adverse reactions of the support.
[0049] B. Adhesion Layers
[0050] Adhesion layers are formed of a material that improves the
adhesion of the interface layer to the support.
[0051] In preferred embodiments, the adhesion layer is of the same
chemical composition as the catalytic layer. In these cases, the
adhesion layer may be designed, in combination with the interface
layer, to reduce migration of the catalytic layer into the
interface layer during nanoparticle synthesis. In some embodiments,
the adhesion layer is iron or an iron alloy. In other embodiments,
the adhesion layer is nickel or a nickel alloy. The adhesion layer
may also be any transition metal, or alloy of that metal, that can
also serve as CNT catalyst.
[0052] In embodiments where the multilayer substrate is employed as
a substrate for the growth of carbon nanotubes, the adhesion layer
must be thick enough to remain as a continuous film at the elevated
temperatures utilized to form CNTs. In certain cases, the adhesion
layer may have a thickness of between about 10 nm and about 150 nm,
more preferably between about 10 nm and about 100 nm, more
preferably between about 10 nm and about 75 nm, most preferably
between about 15 nm and about 50 nm. In certain embodiments, the
adhesion layer has a thickness of about 30 nm.
[0053] The adhesion layer should provide good resistance to oxide
and carbide formation during CNT synthesis at elevated
temperatures. In certain cases, the energy of oxide formation for
the adhesion layer may be greater than -4.5 eV, more preferably
greater than -3.5 eV, most preferably greater than -2.75 eV. In
certain cases, the energy of carbide formation for the adhesion
layer may be greater than -2.5 eV, more preferably greater than
-1.5 eV, most preferably greater than -0.5 eV.
[0054] C. Interface Layers
[0055] In certain embodiments, the interface layer is formed from a
metal which is oxidized under conditions of nanotube synthesis or
during exposure to air after nanotube synthesis to form a suitable
metal oxide. Examples of suitable materials include aluminum,
titanium, gold, copper, silver, and tantalum.
[0056] Alternatively, the interface layer may be formed from a
metal oxide, such as aluminum oxide, silicon oxide, or titanium
dioxide.
[0057] In preferred embodiments, the interface layer is thin enough
to allow the catalytic layer and the adhesion layer to diffuse
across its thickness. In embodiments wherein the catalytic layer
and the adhesion layer have the same composition, this reduces
migration of the catalyst into the interface layer, improving the
lifetime of the catalyst during nanotube growth.
[0058] In certain embodiments, the interface layer has a thickness
of between about 5 nm and about 50 nm, more preferably between
about 7 nm and about 30 nm, most preferably between about 7 nm and
about 15 nm. In certain embodiments, the interface layer has a
thickness of about 10 nm.
[0059] D. Catalytic Layer
[0060] The catalytic layer is typically a thin film formed from a
transition metal that can catalyze the formation of carbon
nanotubes via chemical vapor deposition. Preferably, the catalytic
layer is formed of a material that is resistant to oxidation and/or
carbide formation under the chemical vapor deposition conditions
used to form CNT arrays.
[0061] Examples of suitable materials that can be used to form the
catalytic layer include, but are not limited to, iron, nickel,
cobalt, rhodium, palladium, osmium, iridium, platinum, and
combinations thereof. In particular embodiments, the catalytic
layer contains only materials that catalyze CNT formation, such as
one or more transition metals, including those listed above. In
other embodiments, the catalytic layer materials that catalyze CNT
formation do not contain one or more non-catalytic materials. In
preferred embodiments, the catalytic layer is formed of iron.
[0062] The catalytic layer is of appropriate thickness to aggregate
into small catalytic particles under annealing conditions. The
catalytic layer typically has a thickness of less than about 10 nm.
In preferred embodiments, the catalytic layer has a thickness of
between about 10 nm and about 1 nm, more preferably between about 5
nm and about 1 nm, more preferably between about 2 nm and about 5
nm. In certain embodiments, the catalytic layer has a thickness of
about 3 nm.
[0063] E. Methods of Making
[0064] Multilayer substrates can be formed using a variety of
well-developed techniques for the deposition of metallic thin
films. Non-limiting examples of such techniques include
evaporation, sputter deposition, and chemical vapor deposition. In
some embodiments, the multilayers are formed by sputter deposition
and/or chemical vapor deposition, which can be easier to scale
up.
[0065] Evaporation can be used to deposit thin films of a variety
of metals. The source material to be deposited (e.g., a metal) is
evaporated in a vacuum. The vacuum allows vapor particles to travel
directly to the target object (support), where they condense back
into a solid state, forming a thin film on the target object.
Methods of forming thin films using evaporation are well known in
the art. See, for example, S. A. Campbell, Science and Engineering
of Microelectronic Fabrication, 2.sup.nd Edition, Oxford University
Press, New York (2001). Evaporation typically requires a high
vacuum; however, it is applicable to a variety of metals, and can
deposit metal at rates of up to 50 nm/s. If desired, masks can be
used to pattern the metallic thin films on the target object.
[0066] Metallic and metal oxide thin films can also be formed by
chemical vapor deposition (CVD). Gas precursors containing the
source material to be deposited by CVD (e.g., a metal or metal
oxide) are feed into closed chamber. The chamber can be at
atmospheric pressure or at various grades of vacuum. The chamber
walls can be hot or a heated stage can be used with cold chamber
walls to increase the deposition rate on the target object
(support). Methods of forming thin films using CVD are well known
in the art. See, for example, S. A. Campbell, Science and
Engineering of Microelectronic Fabrication, 2.sup.nd Edition,
Oxford University Press, New York (2001). CVD deposition of metals,
such as iron, aluminum, and titanium, has been demonstrated, so has
CVD deposition of oxides such as aluminum oxide and silicon oxide.
CVD deposition rates can be as low as 1 nm/cycle.
[0067] In one embodiment, electron-beam evaporation is used to form
the multilayer structure on the support. Each layer is deposited at
a pressure less than 0.001 mTorr. The adhesion layer is deposited
at an evaporation rate of 0.3 nm/s. The interface and catalytic
layers are each deposited at an evaporation rate of 0.1 nm/s.
III. CNT Arrays
[0068] CNT arrays contain a plurality of carbon nanotubes which are
vertically aligned on the surface of a material. In some
embodiments, the CNTs are vertically aligned on the multilayer
substrate described above.
[0069] In other embodiments, the CNT arrays are grown on the
multilayer substrates described above by chemical vapor deposition.
In these instances, the process of CNT growth alters the morphology
of the multilayer substrate. Specifically, upon heating or exposure
to air after growth, the interface layer is converted to a metal
oxide, and forms a layer of metal oxide nanoparticles or aggregates
deposited on the adhesion layer. The catalytic layer similarly
forms a series of catalytic nanoparticles or aggregates deposited
on the metal oxide nanoparticles or aggregates. During CNT growth,
CNTs form from the catalytic nanoparticles or aggregates.
[0070] The metal oxide nanoparticles or aggregates typically
contain metal oxide formed from a metal used to form the interface
layer. For example, in embodiments where the interface layer is
formed from aluminum, the metal oxide nanoparticles or aggregates
are formed from aluminum oxide. In embodiments where the interface
layer is formed from a metal oxide, the metal oxide nanoparticles
or aggregates may be composed of the metal oxide used to form the
parent interface layer. The metal oxide nanoparticles or aggregates
may further contain one or more metals which diffuse into the metal
oxide nanoparticles or aggregated from the catalytic layer,
adhesion layer, or combinations thereof. The catalytic
nanoparticles or aggregates may be composed of the metal used to
form the parent catalytic layer.
[0071] The structure of a CNT array grown on the multilayer
substrates described above (200) is shown in FIG. 2. These CNT
arrays contain CNTs (210) anchored to an inert support, preferably
a metal surface, (202) via an adhesion layer (204), metal oxide
nanoparticles or aggregates (206), and catalytic nanoparticles or
aggregates (208).
[0072] Generally, the nanotubes are present at a sufficient density
such that the nanotubes are self-supporting and adopt a
substantially perpendicular orientation to the surface of the
multilayer substrate. In some embodiments, the nanotubes are
oriented, on average, within 30, 25, 20, 15, 10, or 5 degrees of
the surface normal of a line drawn perpendicular to the surface of
the support. Preferably, the nanotubes are spaced at optimal
distances from one another and are of uniform height to minimize
thermal transfer losses, thereby maximizing their collective
thermal diffusivity.
[0073] In some embodiments, the nanotube density on the substrate
surface ranges from about 1.times.10.sup.7 to 1.times.10.sup.11
nanotubes per mm.sup.2, more preferably from about 1.times.10.sup.8
to 1.times.10.sup.10 nanotubes per mm.sup.2, most preferably from
about 1.times.10.sup.9 to 1.times.10.sup.10 nanotubes per
mm.sup.2.
[0074] The CNTs display strong adhesion to the multilayer
substrate. In certain embodiments, the CNT array will remain
substantially intact after being immersed in a solvent, such as
ethanol, and sonicated for a period of at least five minutes.
"Substantially intact" as used herein, means that more than 90%,
91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 99.9% of the CNTs
remained on the surface after sonication, and there was less than
1% change in the thermal resistance of the CNT-multilayer support
interface after sonication. In some embodiments, the thermal
resistance of the CNT-support interface ranges from 1 to 0.1
mm.sup.2K/W, more preferred from 0.5 to 0.1 mm.sup.2K/W, most
preferred from 0.25 to 0.1 mm.sup.2K/W.
[0075] The adhesion of CNT arrays to the substrate can also be
measured using industry standard die shear testing. In this test
the free ends of the CNTs are affixed to another substrate, which
is pushed with controlled force parallel to the substrate until the
CNTs are torn from their interface with the multilayer support. In
some embodiments, the die shear strength of the CNT-multilayer
support interface ranges from 0.2 to 3 MPa, more preferably from
0.5 to 3 MPa, most preferably 1 to 3 MPa.
[0076] In certain embodiments, one or more polymers are applied to
the CNT array. One or more polymers may be adsorbed to the distal
ends of the CNTs to bond the distal ends of the CNTs to a surface,
reduce thermal resistance between the CNT array and a surface, or
combinations thereof. Polymers can be applied to CNT arrays using a
variety of methods known in the art. For example, polymers can be
dissolved in a suitable solvent, and sprayed or spin coated onto
the distal end of the CNTs. A representation is shown in FIG.
3.
[0077] Examples of suitable polymers include conjugated and
aromatric polymers, such as poly(3-hexylthipohene) (P3HT),
polystyrene, and blends thereof. Other examples of suitable
polymers that are neither conjugated nor aromatic include polyvinyl
alcohol (PVA), poly(methyl methacrylate) (PMMA),
polydimethylsiloxane (PDMS), and blends thereof.
[0078] In certain embodiments, one or more metal nanoparticles are
applied to the CNT array. One or more metal nanoparticles may be
adsorbed to the distal ends of the CNTs to bond the distal ends of
the CNTs to a surface, reduce thermal resistance between the CNT
array and a surface, or combinations thereof. Metal nanoparticles
can be applied to CNT arrays using a variety of methods known in
the art. Suitable metal nanoparticles include, but are not limited
to, palladium, gold, silver, titanium, iron, nickel, copper, and
combinations thereof. For example, a solution of metal thiolate
such as palladium hexadecanethiolate can be sprayed or spin coated
onto the distal ends of the CNTs, and the organics can be baked off
to leave palladium nanoparticles. In another example, electron-beam
or sputter deposition can be used to coat metal nanoparticles or
connected "film-like" assemblies of nanoparticles onto the distal
ends of the CNTs.
[0079] In certain embodiments, one or more polymers are applied
together with one or more metal nanoparticles to the CNT array. The
polymers and metal nanoparticles are both adsorbed to the distal
ends of the CNTs to bond the distal ends of the CNTs to a surface,
reduce thermal resistance between the CNT array and a surface, or
combinations thereof. The polymers and metal nanoparticles can be
applied together using a variety of methods known in the art. For
example, a solution of metal thiolate such as palladium
hexadecanethiolate can be sprayed or spin coated onto the distal
ends of the CNTs, and the organics can be baked off to leave
palladium nanoparticles. Then, polymers can be dissolved in a
suitable solvent, and sprayed or spin coated onto the distal ends
of the CNTs that were coated in the previous step with metal
nanoparticles.
[0080] In certain embodiments, flowable or phase change materials
are applied to the CNT array. Flowable or phase change materials
may be added to the CNT array to displace the air between CNTs and
improve contact between the distal ends of CNTs and a surface, and
as a result reduce thermal resistance of the array and the contact
between the array and a surface, or combinations thereof. Flowable
or phase change materials can be applied to CNT arrays using a
variety of methods known in the art. For example, flowable or phase
change materials in their liquid state can be wicked into a CNT
array by placing the array in partial or full contact with the
liquid. A representation is shown in FIG. 4.
[0081] Examples of suitable flowable or phase change materials
include paraffin waxes, polyethylene waxes, hydrocarbon-based waxes
in general, and blends thereof. Other examples of suitable flowable
or phase change materials that are neither wax nor polymeric
include liquid metals, oils, organic-inorganic and
inorganic-inorganic eutectics, and blends thereof.
[0082] In certain embodiments, a liquid is added to the CNT array
and then evaporated to alter the morphology of the array. Capillary
forces that result from liquid evaporation can draw CNTs together
into patterns, which facilitate the addition of flowable or phase
change materials to the array, and/or pull additional CNTs in
contact with a surface, and as a result reduce thermal resistance
of the contact between the array and a surface, or combinations
thereof. Capillary-driven altering of CNT arrays can be
accomplished using a variety of methods known in the art. For
example, solvent can be applied to the CNT array and the array can
be placed in an interface in the wet state and allowed to dry,
activating the capillary forces that ultimately drive CNTs into
contact with the surface. In another example, the CNT array soaked
with solvent can be allowed to dry free from surface contact to
form patterns in the array. A representation is shown in FIG.
5.
[0083] Examples of suitable liquids that can be evaporated from CNT
arrays to change their morphology include solvents such as toluene,
isopropanol, and chloroform, and any other liquid that wets the CNT
arrays sufficiently to penetrate their entire depth.
[0084] A. Carbon Nanotubes
[0085] The CNT arrays contain nanotubes which are continuous from
the top of the array (i.e., the surface formed by the distal end of
the carbon nanotubes when vertically aligned on the multilayer
substrate) to bottom of the array (i.e., the surface of the
multilayer substrate). The array may be formed from multi-wall
carbon nanotubes (MWNTs), which generally refers to nanotubes
having between approximately 4 and approximately 10 walls. The
array may also be formed from few-wall nanotubes (FWNTs), which
generally refers to nanotubes containing approximately 1-3 walls.
FWNTs include single-wall carbon nanotubes (SWNTs), double-wall
carbon nanotubes (DWNTS), and triple-wall carbon nanotubes (TWNTs).
In certain embodiments, the nanotubes are MWNTs. In some
embodiments, the diameter of MWNTs in the arrays ranges from 10 to
40 nm, more preferably 15 to 30 nm, most preferably about 20 nm.
The length of MWNTs in the arrays can range from 1 to 5,000
micrometers, preferably 5 to 5000 micrometers, preferably 5 to 2500
micrometers, more preferably 5 to 2000 micrometers, more preferably
5 to 1000 micrometers.
[0086] B. Methods of Forming CNT Arrays
[0087] In preferred embodiments, the CNTs are grown on the
multilayer substrate using chemical vapor deposition.
[0088] Generally, CNT formation begins by annealing the multilayer
substrate. A suitable carbon source gas is then introduced, and the
temperature is increased to the growth temperature.
[0089] The multilayer substrate is generally annealed for a short
period of time, for example for approximately ten minutes.
Typically, the multilayer substrate is annealed under flow of an
inert gas, such as nitrogen or argon. In certain embodiments, the
annealing temperature is between about 500.degree. C. and about
650.degree. C., more preferably between about 500.degree. C. and
about 600.degree. C., most preferably between about 525.degree. C.
and about 575.degree. C.
[0090] In preferred embodiments, the CNTs are grown on the
multilayer substrate at a growth temperature that is less than the
melting temperature of aluminum (approximately 660.degree. C.). In
certain embodiments, the CNTs are grown on the multilayer substrate
at a growth temperature of between about 600.degree. C. and about
660.degree. C., more preferably between about 610.degree. C. and
about 650.degree. C., most preferably between about 620.degree. C.
and about 640.degree. C. In certain embodiments, the CNTs are grown
on the multilayer substrate at a growth temperature of about
630.degree. C.
[0091] Any suitable carbon source gas may be used. In some
embodiments, the carbon source gas is acetylene. Other suitable
carbon source gases include ethene, ethylene, methane, n-hexane,
alcohols, xylenes, metal catalyst gases (e.g., carbonyl iron), and
combinations thereof. In some embodiments, the source gas is a
metal catalyst gas, which can be used with or without the catalyst
layer.
[0092] In other embodiments, arrays of vertically aligned CNTs are
fabricated on another surface, and transferred, using methods known
in the art, to the distal ends of CNTs on the multilayer substrate.
For example, a CNT array that is 5 micrometers or shorter is grown
on the multilayer substrate. Then a very tall CNT array, around 500
micrometers in length, is transferred distal-end-to-distal-end onto
the short CNTs adhered to the multilayer substrate. The distal ends
of the two CNT arrays are bonded by polymers, metal nanoparticles,
or a combination of both by coating the distal ends with such
before the transfer. This technique is referred to as transfer
printing. In the case of metal nanoparticle bonding, the CNT arrays
and multilayer substrate are heated to promote metal diffusion and
to secure the bond. As an example, the heating is done at
300.degree. C. in air for 30 min to and 1 hour; and the two CNT
arrays are placed under 20 to 40 psi of pressure during
heating.
IV. Methods of Use
[0093] The CNT arrays described herein can be used as thermal
interface materials. The CNT arrays can be formed and/or deposited,
as required for a particular application.
[0094] For example, in one embodiment, the inert support for the
multilayer substrate and CNT arrays is a piece of metal foil, such
as aluminum foil. In these cases, CNTs are anchored to a surface of
the metal foil via an adhesion layer, metal oxide nanoparticles or
aggregates, and catalytic nanoparticles or aggregates. In some
instances only one surface (i.e., side) of the metal foil contains
an array of aligned CNTs anchored to the surface. In other cases,
both surfaces (i.e., sides) of the metal foil contain an array of
aligned CNTs anchored to the surface. If desired, one or more
polymers, metal particles, or combinations thereof may be applied
to the CNT array.
[0095] These materials may be placed or affixed in between a heat
source and a heat sink or heat spreader, such as between an
integrated circuit package and a finned heat exchanger, to improve
the transfer of heat from the heat source to the heat sink or
spreader.
[0096] CNT arrays of this type exhibit both high thermal
conductance and mechanical durability. As a consequence, these
arrays are well suited for applications where repeated cycling is
required. For example, foils of this type can be employed as
thermal interface materials during turn-in testing of electrical
components, such as chips.
[0097] In other embodiments, the inert support for the multilayer
substrate and CNT arrays is a surface of a conventional metal heat
sink or spreader. In these cases, CNTs are anchored to a surface of
the heat sink or spreader via an adhesion layer, metal oxide
nanoparticles or aggregates, and catalytic nanoparticles or
aggregates. This functionalized heat sink or spreader may then be
abutted or adhered to a heat source, such as an integrated circuit
package.
[0098] The CNT arrays described herein can be used as thermal
interface materials in personal computers, server computers, memory
modules, graphics chips, radar and radio-frequency (RF) devices,
disc drives, displays, including light-emitting diode (LED)
displays, lighting systems, automotive control units,
power-electronics, solar cells, batteries, communications
equipment, such as cellular phones, thermoelectric generators, and
imaging equipment, including MRIs.
[0099] The CNT arrays can also be used for applications other than
heat transfer. Examples include, but are not limited to,
microelectronics, through-wafer vertical interconnect assemblies,
and electrodes for batteries and capacitors. Currently, copper and
aluminum foil are used as the backing materials for the anode and
cathode in lithium ion batteries. A slurry of activated carbon and
the lithium materials is pasted onto the foils. The electrical
contact between the paste and the foil is a point of parasitic
resistance. In addition to reduced electrical output this
resistance can impede heat rejection from the device. Well adhered
vertical CNT arrays placed at this interface would improve
performance electrically and thermally.
[0100] The CNT foils could also be used for electromagnetic
shielding. The CNTs act to effectively absorb electromagnetic
irradiation as well as solar absorbing material, to enhance solar
absorption in solar hot water heaters.
Examples
Example 1
Preparation of Carbon Nanotube (CNT) Arrays
[0101] Aluminum foil was purchased at a thickness of 10 micrometers
from Alfa Aesar. A piece of aluminum foil was placed in a square
sample holder in a Denton Explorer electron-beam evaporator. The
sample holder clamped the aluminum foil around its edges and a
5.times.5 inch square of the aluminum foil was exposed on the
front- and backside of the sample holder, which could be flipped
in-situ to deposit metal on both sides of the foil without breaking
vacuum.
[0102] One side at a time, an adhesion layer of iron was deposited
to a thickness of 30 nm, then an interface layer of aluminum was
deposited to a thickness of 10 nm, and finally a catalytic layer of
iron was deposited to a thickness of 3 nm. The aluminum layer was
allowed to cool for 10 minutes before depositing the catalytic iron
film. The depositions all occurred at a chamber pressure of
approximately 0.0008 mTorr. The iron adhesion layers were deposited
at a rate of 0.1 nm/s; the aluminum interface layers were deposited
at a rate of 0.1 nm/s; and the iron catalytic layer was deposited
at a rate of 0.05 nm/s. The deposited multilayer substrates were
allowed to cool for 15 min before venting the chamber and removing
the aluminum foil.
[0103] An Aixtron Black Magic CVD tool was used to grow CNTs on the
multilayer substrates. The aluminum foil with multilayers on both
sides was placed on a stage in the CVD tool. The sample was heated
in a nitrogen atmosphere at 10 Torr to a temperature of 550.degree.
C., and then the sample was annealed at this temperature for 10
minutes in nitrogen at 10 Torr. Hydrogen was fed into the chamber
at the end of the nitrogen annealing step and the sample was held
at the annealing temperature for an additional 3 minutes in the
nitrogen and hydrogen atmosphere. Acetylene was introduced to the
chamber and nitrogen flow was stopped at the end of the 3 minutes,
and then the sample was heated to 630.degree. C. CNT growth
commenced for 5 minutes at 630.degree. C. and 10 Torr with 700
standard cubic centimeters per minute (sccm) of hydrogen and 100
sccm of acetylene as process gasses. Hydrogen and acetylene gas
flow was stopped at the end of 5 minutes and the aluminum foil with
CNTs adhered via multilayers was allowed to cool to 200.degree. C.
in a nitrogen flow.
[0104] Dense vertical CNT arrays approximately 12 micrometers tall
were produced on the side of the aluminum foil facing up, and dense
vertical CNT arrays approximately 10 micrometers tall were produced
on the side of the aluminum foil facing the sample stage. The
densities of CNTs on both sides of the foil were determined by
scanning electron microscopy (SEM) to be about 1.times.10.sup.9
nanotubes per mm.sup.2. The diameters of the CNTs on both sides of
the foil were determined by SEM to be about 10 nm. The produced
CNTs were MWNTs, which had 5 walls on average.
[0105] The produced aluminum foil sample with CNTs adhered with
multilayers on both sides was placed in a sonication bath of
ethanol for 5 minutes. No CNTs were observed to release from the
substrate during the sonication, which demonstrates the excellent
adhesive and cohesive integrity of the joint. Upon removal from the
ethanol, the CNTs in the array were patterned into discrete
islands, demonstrating that solvent evaporation from the array can
be an effective method to alter the morphology of the array.
[0106] The distal ends of the CNT arrays on both sides of the
aluminum foil were coated with P3HT by spray coating. The structure
is shown in FIG. 6. The polymer-coated sample was pressed at 20 psi
between gold-coated silver and quartz surfaces that were wet with
chloroform. The interface was allowed to dry and the thermal
resistance was measured using a photoacoutic technique. The thermal
resistance was estimated to be approximately 7 mm.sup.2K/W, which
is a 70% reduction in resistance compared to the sample structure
tested without the polymer coating.
Example 2
Preparation of Carbice Carbon Nanotube (CNT) Arrays Using First
Nano CVD System
[0107] Thermal CNT Growth using First Nano CVD System
[0108] Thermal CNT growth was performed in a First Nano Easy Tube
Chemical Vapor Deposition (CVD) furnace at sub-atmospheric
pressures (.about.300-400 torr) with H.sub.2, C.sub.2H.sub.2 as the
growth gases. CNT growth was performed on Al and Cu foils in this
furnace using the Carbice Fe30/Al10/Fe3 nm catalyst (multi-layer
substrate) and some variations to the catalyst as described in the
following sections.
[0109] CNT Growth on Al Foils Using Carbice Catalyst (Fe30/Al10/Fe3
nm)
[0110] CNT growth was performed on 25 .mu.m thick Al foil in the
First Nano CVD furnace using the following low pressure chemical
vapor deposition (LPCVD) procedure at 630.degree. C. The sample was
placed in the CVD furnace and the temperature increased to
530.degree. C. in Ar at 400 sccm. The sample was annealed in
H.sub.2 at 350 sccm for 3 mins. C.sub.2H.sub.2 was then introduced
into the chamber at 50 sccm. The temperature was increased to
630.degree. C. with the sample in H.sub.2 at 350 sccm and
C.sub.2H.sub.2 at 50 sccm. CNT growth commenced for 20 mins in
H.sub.2 at 350 sccm and C.sub.2H.sub.2 at 50 sccm at 630.degree. C.
at -330 torr pressure.
[0111] Results
[0112] Under these growth conditions, a fully densified array of
CNTs was produced. The CNTs were approximately 17 micrometers tall,
vertically aligned and well adhered to the substrate. Dry contact
thermal resistances for CNTs grown under these conditions, measured
in a stepped 1D reference bar apparatus, was about 1.6
cm.sup.2-K/W. Sample variability in CNT heights for this growth
time is dependent on the temperature distribution and flow
conditions inside of the furnace as well as the quality of the
deposited catalyst. Typical CNT heights for this growth time range
from about 15-25 micrometers. Shorter CNTs (.about.5-7 micrometers)
were grown by reducing the growth times with the same catalyst and
growth gases. Under this condition, CNTs with thermal resistances
that were smaller by a factor of three or more were produced.
[0113] CNT growth on Al and Cu Foils using Modified Carbice
Catalyst (Fe10/Al100Fe30/Al10/Fe3 nm)
[0114] The CNTs heights can be increased or decreased by changing
the growth time in kind, however growth typically terminates at
about 50-60 micrometers, at which point increasing growth time does
not continue to increase CNT height. Because the growth termination
mechanism is partially related to sub surface diffusion, a slightly
modified five layer catalyst system was implemented to combat the
diffusion process. Using a catalyst of Fe10/Al100/Fe30/Al10/Fe3 nm,
CNTs of 75-100 micrometers in height were grown on a 50 micrometer
Al substrate with 45 minute growth time. The other growth
parameters (e.g. growth gases, temperature, etc.) remain the same
as described above. This modified catalyst represents a double
stacking of the Carbice catalyst with thickness modifications in
the first two layers. The first Fe layer serves as an adhesion
promoter for the rest of the catalyst stack, and the Al acts as a
diffusion barrier. In addition to the increased relative diffusion
distance associated with the modified Carbice catalyst, the
additional interfaces also provided some resistance to interlayer
diffusion.
[0115] Catalyst poisoning due to Cu diffusion from Cu substrates is
much more problematic than the analogous problem seen when growing
on Al substrates. For this reason, the standard three-layer Carbice
catalyst (Fe30/Al10/Fe3 nm) is often not sufficient for repeatable
growth on Cu. The modified 5 layer Carbice catalyst
(Fe10/Al100/Fe30/Al10/Fe3 nm) overcomes this problem while allowing
repeatable growth on Cu up to very tall CNT heights.
[0116] For example, using the Fe10/Al100/Fe30/Al10/Fe3 nm catalyst,
150 micrometers CNTs were grown on an oxygen-free high conductivity
copper substrate with a 90 minute growth time at 650.degree. C. The
higher melting temperature of Cu allows for this slight increase in
growth temperature. The growth gases and ramp rates remain the same
as described above.
Example 3
Performance Study of Carbice TIM Compared to Grafoil.RTM.
[0117] The following experiment determines the impact of the
CarbiceTM product in a potential end use application compared to a
competing product (Grafoil.RTM.).
[0118] The output voltage of a thermoelectric module was measured.
Ten (10) baseline measurements using Grafoil.RTM. were performed.
Ten (10) measurements with Carbice CNT TIM both on copper and
aluminum substrate were also performed. All measurements were
performed at hot side temperature set points of 30, 60, 90, 120,
and 180.degree. C. All tests were performed on a Marlow Industries
RC3-6 Bi.sub.2Te.sub.3 thermoelectric module.
[0119] Results
[0120] A comparative curve of CNT on copper, CNT on aluminum, and
Grafoil.RTM. is shown in FIG. 7. A summary of the results is shown
in Table 1. Under these test conditions, Carbice TIM significantly
improves system performance compared to conventional graphite-based
TIMs. Specifically, output voltages increased 20% or more for
Copper and Aluminum substrates compared to Grafoil.
TABLE-US-00001 TABLE 1 Summary of comparative study. Improvement
Interface Voltage (over baseline) Grafoil (baseline) 1.148 --
Carbice TIM Copper Substrate 1.383 20% .uparw. Carbice TIM
Aluminium Substrate 1.465 28% .uparw.
* * * * *